The NMR spectroscopist often finds it .necessary to observe a wide variety of nuclides, especially .sup.13 C, .sup.1 H, .sup.19 F, .sup.27 Al, .sup.29 Si, .sup.2 H, and .sup.15 N in the study of commercially and scientifically important chemicals. There have been numerous applications for double resonance circuits for the past three decades. The most common application is irradiating at the proton (.sup.1 H) high frequency (HF) resonance to decouple its dipolar broadening effects while observing Bloch decays on a nuclide of lower magnetogyric ratio such as .sup.13 C at a low frequency (LF). Other examples include cross-polarization and inverse detection. Broadband (multinuclear) circuits as shown in FIG. 1 have provided multinuclear tuning capability on both channels so that cross polarization could be performed between any two of a large number of different nuclides.
For experiments on solid samples at high static magnetic field B.sub.0 (greater than 6 T) , where large RF fields B.sub.1 are required (greater than 0.6 mT), typical voltages across the sample coil are 2 to 6 kV. In F. D. Doty, T. J. Connick, X. Z. Ni, and M. N. Clingan, J. Mag. Res. 77, "Noise in High Power, High Frequency Double Tuned Probes," 536 (1988), the authors discuss the various requirements for a high-performance, high-field, double-tuned solids probe for NMR. In U.S. Pat. No. 5,162,739, I disclose an efficient method of balancing a double-tuned high-power circuit so that proton B.sub.1 can exceed 2 mT at 400 MHz with 0.3 ml samples in a Cross Polarization Magic Angle Spinning (CPMAS) probe that is broadbanded on the LF channel and uses a sample spinner such as the one described in U.S. Pat No. 5,202,633.
It has sometimes been useful to be able to perform CP experiments pair-wise between three or four nuclides under MAS conditions, where it is difficult to use orthogonal sample coils. Fixed-frequency triple-resonance and quad-resonance single-sample-coil circuits suitable for high-power NMR experiments have been commercially available for eight years. Most have been similar to the triple-resonance circuit shown in FIG. 2, which can be extended with another inductor and set of tuning elements to quad-resonance. The trap-matrix approach of Schnall et al, U.S. Pat. No. 4,742,304, is effective when sample losses dominate other loss mechanisms, as in many MRI situations. The method of McKay, U.S. Pat. No. 4,446,431, using transmission line sections, generally results in degraded transient response for solids applications but it provides more flexibility in the selection of variable capacitors. McKay's method has also been extended to triple and quad-resonance, and others have applied it to MRI applications.
Recently, several new triple-resonance techniques have been developed that have considerable value in determining internuclear distances to a high level of precision in complex molecules. One such technique, that has (oddly) been referred to as Rotational Echo Double Resonance (REDOR), is described in "Determination of the Molecular Conformation of Melanostatin Using .sup.13 C-.sup.15 N-REDOR NMR Spectroscopy," by J. R. Garbow and C. A. McWherter, in the Journal of the American Chemical Society, p. 238, 1993. The REDOR technique appears to have considerable value in drug design and other biochemical analytical applications. This technique requires a triple resonance circuit in which the HF channel is tuned for .sup.1 H decoupling and the LF and mid-frequency (MF) channels are tuned to the respective frequencies of two other nuclides whose internuclear distance is to be determined--for example, .sup.13 C-.sup.15 N, .sup.31 P-.sup.23 Na, .sup.31 P-.sup.15 N etc. Hence, it is desirable to provide a triple resonance circuit that is broadbanded at both the LF and MF frequencies.
The difficulties involved in simultaneously achieving high efficiency, adequate channel isolation, simple tune-up, and high power capability in a triple resonance circuit within the constraints of a typical NMR magnet have heretofore prevented the realization of a doubly broadband triple-resonance single-sample-coil NMR circuit by any combination of the prior art, as will be described.
Many factors conspire against the would-be designer of an NMR probe. A workable NMR probe has to fit in the close confines of the magnet being used; it has to be mechanically robust in the face of gravitational and magnetic forces, ruling out many materials from which it might otherwise be constructed; its components must tolerate the high voltages and strong RF fields that are present; and stray inductances and capacitances have to be controlled.
While any NMR probe design is difficult, a doubly broadband triple or quad resonance NMR probe circuit is particularly difficult. While a schematic design is easy to develop which would theoretically satisfy the probe functional needs, many schematic designs turn out to be impractical physically. For example, some of the tuning capacitors used in probes are physically large and require substantial mounting arrangements. If one end of a capacitor is to be grounded, the grounded connection can be a mechanical connection as well, providing the mounting means for the capacitor. But if neither end is to be grounded, some way must be found to mount the capacitor in an insulated fashion. If both ends of the capacitor are to be at high voltage (and if the capacitor is a large one) then it is quite difficult to work out a way to mount the capacitor robustly.
While it is an easy matter to rule out various proposed circuit schematics due to the inability of their actual construction in a workable probe, it is no easy matter to arrive at a circuit schematic that can actually be built, will fit within the space allowed, and will work. The key concepts disclosed herein enabling the doubly broadband triple resonance circuit are also applicable to doubly-broadband quad-resonance circuits for NMR techniques requiring four high-power channels.